Combustion Wave Structures

The combustion wave of a double-base propellant consists of the following five successive zones as shown in Fig. 6-3, (I) heat conduction zone, (II) solid phase reaction zone, (III) fizz zone, (IV) dark zone, and (V) flame zone 10 13 14  

The solid phase reaction zone is also termed the subsurface reaction zone or condensed phase reaction zone. The dark zone reaction is the induction zone to produce the flame zone. Thus, the dark zone is also termed the preparation zone to produce the luminous flame. Since the flame zone is luminous, it is also termed the luminous flame zone . 

The thermal structure of the combustion wave of double-base propellants is understood from the temperature profile traces in the combustion wave. In the solid phase reaction zone, the temperature in the solid phase increases rapidly from the initial temperature T0 to the onset temperature of the solid phase reaction zone, Tu 

The reaction time to produce the luminous flame, xd, is given by 

The heat flux feedback from the gas phase to the condensed phase increases as (N02) increases, and the burning rate thus increases as the energy density of double-base propellants increases as shown in Figs. 6-1 and 6-2. 

The burning rates of a nitro-azide propellant composed of NC, NG, and GAP are shown in Fig. 6.19. For comparison, the burning rates of a double-base propellant composed of NC, NG, and DEP are shown in Fig. 6.20. The chemical compositions of both propellants are shown in Table 6.6. The adiabatic flame temperature is increased from 2560 K to 2960 K and the specific impulse is increased from 237 s to 253 s when 12.5% of DEP is replaced with the same amount of GAP. 

Though both propellants contain equal amounts of NC and NG, the burning rate of NC-NG-GAP is approximately 70% higher than that of NC-NG-DEP at Tq = 293 K. The pressure exponent of burning rate remains relatively unchanged at = 0.7 by the replacement of DEP with GAP. However, the temperature sensitivity of burning rate defined in Eq. (3.73) is increased significantly from 0.0038 K to 0.0083 K-  

The combustion wave of an NC-NG-GAP propellant consists of successive two-stage reaction zones.0 1 The first gas-phase reaction occurs at the burning surface and the temperature increases rapidly in the fizz zone. The second zone is the dark zone, which separates the luminous flame zone from the burning surface. Thus, the luminous flame stands some distance above the burning surface. This structure 

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A schematic representation of the combustion wave structure of a typical energetic material is shown in Fig. 3.9 and the heat transfer process as a function of the burning distance and temperature is shown in Fig. 3.10. In zone I (solid-phase zone or condensed-phase zone), no chemical reactions occur and the temperature increases from the initial temperature (Tq) to the decomposition temperature (T ). In zone II (condensed-phase reaction zone), in which there is a phase change from solid to liquid and/or to gas and reactive gaseous species are formed in endothermic or exothermic reactions, the temperature increases from T to the burning surface temperature (Tf In zone III (gas-phase reaction zone), in which exothermic gas-phase reactions occur, the temperature increases rapidly from Tj to the flame temperature (Tg). 

The combustion wave structure of ADN consists of three zones the melt layer zone, the preparation zone, and the flame zone. The temperature remains relatively unchanged in the melt layer zone, then increases rapidly just above the melt layer zone to form the preparation zone, in which it rises from about 1300 to 1400 K. At some distance above the melt layer zone, the temperature increases rapidly to form the flame zone, in which the final combustion products are formed. 

The combustion wave structure for HNF consists of two gas-phase zones, similar to that for ADN. However, the melt layer zone observed for ADN is not seen for HNF. 

Thus, the combustion wave structure of double-base propellants appears to showa two-stage gas-phase reaction, taking place in the fizz zone and the dark zone. The thickness of the fizz zone is actually dependent on the chemical kinetics of the... 

Fig. 6.15 Combustion wave structure of a double-base propellant at different initial propellant temperatures and at high and low pressures.

In order to clarify the combustion wave structure of AP composite propellants, photographic observations of the gas phase at low pressure are very informative. The reaction rate is lowered and the thickness of the reaction zone is increased at low pressure. Fig. 7.3 shows the reduced burning rates of three AP-HTPB composite propellants at low pressures below 0.1 MPa.FI The chemical compositions of the propellants are shown in Table 7.1. The burning rate of the propellant with the composition ap(0-86) is higher than that of the one with ap(0-80) at constant pressure. However, the pressure exponents are 0.62 and 0.65 for the ap(0-86) and Iap(0.80) propellants, respectively that is, the burning rate is represented by r for the p(0.86) propellant and by r for the p(0.80) propellant. 

Combustion Wave Structure of Oxidizer-Rich AP Propellants... 

The combustion wave structure of RDX composite propellants is homogeneous and the temperature in the solid phase and in the gas phase increases relatively smoothly as compared with AP composite propellants. The temperature increases rapidly on and just above the burning surface (in the dark zone near the burning surface) and so the temperature gradient at the burning surface is high. The temperature in the dark zone increases slowly. However, the temperature increases rapidly once more at the luminous flame front. The combustion wave structure of RDX and HMX composite propellants composed of nitramines and hydrocarbon polymers is thus very similar to that of double-base propellants composed of nitrate esters.[1 1... 

Though the physicochemical properties of HTPE and HTPS are different, both are subject to a similar super-rate burning effect. However, the magnitude of the effect is dependent on the type of binder used. As in the case of double-base propellants, the combustion wave structures of the respective propellants are homogeneous, even though the propellant structures are heterogeneous and the luminous flames are produced above the burning surfaces. 

The combustion wave structure of HMX propellants catalyzed with LiF and C is similar to that of catalyzed nitropolymer propellants the luminous flame stands some distance above the burning surface at low pressures and approaches the burning surface with increasing pressure. The flame stand-off distance from the burning surface to the luminous flame front is increased at constant pressure when the propellant is catalyzed. The flame stand-off distance decreases with increasing pressure for both non-catalyzed and catalyzed propellants. 

The combustion wave of an HMX composite propellant consists of successive re-achon zones the condensed-phase reachon zone, a first-stage reaction zone, a second-stage reaction zone, and the luminous flame zone. The combustion wave structure and temperature distribution for an HMX propellant are shown in Fig. 7.47. In the condensed-phase reaction zone, HMX particles melt together with the polymeric binder HTPE and form an energetic liquid mixture that covers the burning surface of the propellant. In the first-stage reaction zone, a rapid exother-... 

Fig. 7.47 Combustion wave structures of non-catalyzed and catalyzed HMX composite propellants.

Kubota, N., Kuwahara, T., Miyazaki, S., Uchiyama, K., and Hirata, N., Combustion Wave Structure of Ammonium Perchlorate Composite Propellants, Journal of Propulsion and Power, Vol. 2, No. 4,... 

Fig. 8.6 Temperature profiles in the combustion wave structures of an RDX-CMDB propellant at different pressures.

Aoki, I., and Kubota, N., Combustion Wave Structure of HMX-CMDB Propellants (Eds. Varma, M., and Chatterjee, A. K.), pp. 133-143, Tata McGraw-Hill Publishing Co., Ltd., India (2002). 

Measurements of Burning Rate and Combustion Wave Structure... 

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